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Journal of Physics B Atomic Molecular and Optical Physics

Dispersive shaping of a single-photon pulse in a lossless medium of cold atoms

Sh. Petrosyan

Published Sep 24, 2014• 0 citations
Publisher

Abstract

We demonstrate reshaping of a short single-photon (SP) pulse coherently propagating in a lossless medium of cold Λ-type atoms in far off-resonant Raman configuration. We show the sole responsibility of dispersive effects for different temporal behavior of the SP pulse ranging from a slow photon to oscillating photon regime, as well as for programmable temporal modulation of photons by using control pulses with different frequency chirping.

DOI
10.1088/0953-4075/47/19/195502

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We demonstrate reshaping of a short single-photon (SP) pulse coherently propagating in a lossless medium of cold Λ-type atoms in far off-resonant Raman configuration. We show the sole responsibility of dispersive effects for different temporal behavior of the SP pulse ranging from a slow photon to oscillating photon regime, as well as for programmable temporal modulation of photons by using control pulses with different frequency chirping.
Single-photon coherent optics represents a fundamental importance for the investigation of quantum light-matter interactions. While most work has considered the interaction in the steady-state regime, here we demonstrate that a single-photon pulse shorter than any relaxation time in a medium propagates without energy loss and is consistently transformed into a zero-area pulse. A general analytical solution is found for photon passage through a cold ensemble of $\ensuremath{\Lambda}$-type atoms confined inside a hollow core of a single-mode photonic-crystal fiber. We use the robust far-off-resonant Raman scheme to control the pulse reshaping by an intense control laser beam and show that in the case of cw control field, for exact two-photon resonance, the outgoing photon displays an oscillating temporal distribution, which is the quantum counterpart of a classical field ringing, while for nonzero two-photon detuning a slow photon is produced. We demonstrate also that a train of readout control pulses coherently recalls the stored photon in many well-separated temporal modes, thus producing time-bin entangled single-photon states. Such states, which allow sharing quantum information among many users, are highly demanded for applications in long-distance quantum communication.
A quasi-transparent propagation phenomenon is presented for a pair of frequency-modulated laser pulses in an optically thick medium of cold atomic gas. The noninteracting, identical lambda-structured atoms are driven by two classical laser fields, which are frequency-chirped in the same way around the corresponding atomic transition frequencies, maintaining two-photon (Raman) resonance. It is shown by numerical analysis that after propagating over a relatively short distance (determined by the absorption length), the frequency-chirped pulse pair is affected by the atoms in such a way that instead of exciting the atoms (as it would happen in an optically dilute medium), they create a certain coherent superposition of the ground states (which can be varied by the parameters of the incoming fields), and they propagate in the remainder of the medium without significant further losses. This quasi-lossless propagation effect of the Raman-resonant frequency-chirped pulses, described above, is not only interesting in the point of view of the laser fields, but the on-demand creation of coherent superpositions among atomic states along the optically thick medium may also find applications in quantum optical experiments and quantum informatics.
The light wave propagating in a dispersive medium can be formularized as electromagnetic (EM) fields or quantized as a pair of Hermitian operators. The fascinating optical phenomena display classical and nonclassical properties, depending on whether they can be described by the classical EM field theory. A cold atomic ensemble with neligible inhomogenous Doppler broadening is ideal to explore the interaction between light and matter. A typical problem in classical pulse propagation is optical transients, which induced by rapid response of dispersive medium to the sudden change of the driving radiation fields. With cold atoms confined in the two-dimensional magneto-optical trap, whose optical depth is controllable in our system, optical precursor and free-induction decay both classified as optical transients are observed in a single experiment with different conditions. Moreover, the information velocity limit is verified in superluminal medium, with the observation and analysis of precursors. Characterized with narrow linewidth and long coherence time, paired photons generated from cold atomic ensemble in four-wave mixing process possess irreplaceable advantage in quantum optics area. Temporal modulation of narrow-band paired photons can be achieved by directly modulating the generated photons or by modulating the classical control fields. After successfully reshaping the temporal waveform of the paired photons, heralded single photon source is well prepared for studying the propagation of nonclassical single photon in dispersive medium. The speed limit imposed by Einstein Causality principle is firstly experimentally verified on single photon level.
The spatial and temporal dynamics of two short pulses propagating in an optically dense medium of resonant three-level Λ-atoms is investigated numerically and analytically. The maximum coherence for the Raman transition due to coherent population trapping. It is shown that, at the initial stage of propagation, the waveforms of such pulses only slightly change along the length of the medium, which may considerably exceed the length of linear absorption for a single weak pulse. As the length of the absorbing medium increases, the energy of the probe (first) pulse is completely transferred into the second (control) pulse.
Narrow-band time-frequency entangled biphotons are generated from spontaneous four-wave mixing in cold atom clouds. The coherence time of the entangled biphotons can be extended to sub-microseconds by the slow light effect. The temporal wavefunction of the biphotons can be manipulated through modulating the spectral or spatial mode of the controlling laser beams. Concerning a pair of entangled biphoton and the resulting heralded single photon, it was commonly believed that, time-frequency entanglement damages the temporal purity of the single photon. However, the case is totally different for biphotons with long coherence time which is far beyond the time resolution of single-photondetectors. We demonstrate that, the single photon heralded from these time-frequency entangled biphotons is in a pure temporal state. Therefore, single photons are able to be shaped through the time-frequency entanglement with their partner photons, while the single photons could be found to be still in a pure state and thus useful for quantum information processing and communication technology.